Electron tube



March 9, 1937. .J. D. MCGEE 2,072,957

' ELECTRON TUBE Filed May 26, 1933 INVENTOR JAMES DWYER Mc GEE BY 71% W-ATTO NEY Patented Mar. 9, 1937 UNITED STATES PATENT OFFICE ELECTRONTUBE Application May 26, 1933, Serial No. 672,974 In Great Britain June2, 1932 Claims.

The present invention relates to cathode ray tubes and more particularlyto means for concentratingand defining the electron ray emitted by thecathode.

In many of the systems in which cathode ray tubes are at present used itis desirable that the ray itself should be of as high a concentration aspossible whilst its cross-sectional area should be as small as possible.These conditions must be fulfilled, for example, if the ray be used forpurposes of scanning in a television receiving system in order toproduce upon a fluorescent screen a visible spot of high brilliancy andsmall size.

It has already been proposed to concentrate the ray either by means of amagnetic field produced at the centre of current carrying coils throughwhich the ray is passed, or by means of an electrostatic field producedbetween two apertured plane electrodes placed normally to the meandirection of the ray. It has also been proposed to limit thecross-sectional area of the ray by means of stops of suitable sizeplaced in the path of the beam.

It has been found, however, that the focussing effect produced by thesearrangements is comparatively inefficient and that in consequence alarge electron loss occursat those stops designed to reduce thecross-sectional area of the beam, such a lossbeing particularlynoticeable if the electronemitting surface of the cathode is ofcomparatively large area.

It is an object of the present invention to avoid the above describedloss of electrons at limiting stops, by providing improved means forconcentrating the ray into a narrow pencil of electrons.

According tothe presentinvention a cathode ray tube comprises anapertured control electrode, two or more apertured auxiliary electrodesand an anode arranged in the tube in the order mentioned, wherein one ormore of said auxiliary electrodes is in the form of an annulus having afrusto-conical or other dished shape.

According to a further feature of the present invention a cathode raytube comprises an apertured control electrode, two or more aperturedauxiliary electrodes and an anode arranged in the tube'in the ordermentioned, wherein the sizes of the apertures in said aperturedelectrodes are dependent upon the positions of the electrodes relativeto the cathode, being smaller the greater the distance of the electrodefrom the cathode.

A constructional embodiment of the invention will now be described, byway of example, with reference to the accompanying drawing, in which:

Fig. 1 illustrates, diagrammatically, anv arrangement of concentratingelectrodes within a cathode ray tube according to the present invention;

Fig. 2 illustrates, diagrammatically, and in greater detail, some of theelectrodes shown in Fig. l and the electrostatic lines of force betweenthem; and

Fig. 3 illustrates, also diagrammatically, an

.alternative arrangement of concentrating electrodes within a cathoderay tube according to this invention.

Referring now to Fig. 1, a cathode ray tube comprises a sealed glassenvelope l a portion 2 of which is of cylindrical shape closed at oneend and another portion 3 of which is of frustoconical shape merginginto the open end of the cylindrical part. Upon the base of thefrustoconical portion 3 is deposited at fluorescent screen 4. Theindirectly heated cathode 5, which for the moment will be assumed to beof small emitting area, is situated near the closed end of thecylindrical portion 2. The anode 6 is deposited in the form of ametallic coating upon the inner surface of the cylindrical portion 2,and may, if desired, extend somewhat into the open end of thefrusto-conical portion 3. v

The general direction of the ray is thus along the axis of the tube fromthe cathode 5 to the centre of the screen 4 on the base of thefrustoconical portion 3.

Close to the cathode 5 is placed a control electrode 1 which is in theform of a cylinder having a plane metallic diaphragm which is disposednormally to the mean direction of the ray and.

which has a central aperture 8 large enough to allow a high percentageof the electrons emitted from the cathode 5 to pass through. At a smalldistance from the control electrode 1 is placed an auxiliary electrode 9which is in the form of a metallic frusto-conical annulus with itssmaller diameter end It! pointing toward the cathode 5, and close to theauxiliary electrode is placed a second auxiliary electrode I l which isin the form of a metallic cylinder having two diaphragms with aperturesi2 and I3. Beyond the second auxiliary electrode II is disposed theanode 6, which as already stated is in the form of a metallic deposit onthe walls of the tube.

The potentials applied to the electrodes, relative to the cathode,increase positively and progressively in the order 9, ll and B, that isto say, the potentials of the electrodes, relative to the cathode, aregreater the greater the distance of the electrodes from the cathode.With this arrangement the electrons extracted from the cathodeexperience an accelerating force from the cathode to the anode and onreaching the cylindrical anode the electrons are projected through thecylinder on to the fluorescent screen 4.

The electrostatic lines of force between the cathode and the first andsecond auxiliary electrodes are illustrated in Fig, 2. It will be seenthat in the neighborhood of the cathode the lines of force are directedtowards the axis of the tube and that between the control electrode andthe anode the lines tend to crowd on the axis. An electron leaving thecathode is thus urged on to the axis of the tube and, once there, has tocross a line of force in order to diverge from the axis; if it succeedsand diverges outwards it again passes through a field urging it back onto the axis. As a result of this efiicient concentration of the rayalong the axis of the tube, the sizes of the limiting apertures 8, l0,l2, and I3 in the electrodes l, 9, and H can be made dependent upon thepositions of the electrodes with respect to the cathode. The aperturescan, in fact, be made smaller the greater their distance from theoathode without seriously affecting the total number of electronsreaching the fluorescent screen 4.

If the tube be used for purposes of scanning in a television system,modulation of the intensity of the ray is effected by varying thepotential applied to the control electrode 1 relative to the cathode 5.In practice it has been found convenient to earth the cathode and biasthe control electrode at about ten volts negative. The potentialsapplied to the remaining electrodes,

' which may be derived from a suitable arrangement of high resistancesand one source of high potential, are as follows:-

The frusto-conical electrode is maintained at about two to three hundredvolts positive, the second auxiliary electrode at one thousand voltspositive and the anode at four thousand volts positive.

The concentrating efiect is most eflicient when the ratio VC' VB D- c'is equal to or less than the ratio where VB, V0 and VD represent thepotentials applied to the control, the frusto-conical and the secondauxiliary electrodes respectively, relative to the cathode, 0 representsthe shortest distance between the control electrode and thefrustoconical electrode, and 11 represents the shortest distance betweenthe apex of the frusto-conical electrode and the second auxiliaryelectrode.

The electrodes are held in position and adjusted to be accuratelyco-axial by a suitable arrangement of metal and glass struts (not shown)and insulating spacers l4.

If desired the concentrating efiect can be intensified by arrangingco-axially in the tube two or more frusto-conical electrodes. Such anarrangement, which is illustrated in Fig. 3, is of particular advantageif the source of electrons is of large emitting area compared to thecrosssectional area of the ray desired.

As before, there is arranged, close to the cathode 5, a plane controlelectrode 1 in the form of a metallic plate having a sufliciently largecentral aperture 8 to accommodate substantially all of the electronsemitted from the large cathode 5. On the other side of the controlelectrode 1 from the cathode 5 there is arranged a first auxiliaryelectrode 9 in the form of a frustoconical annulus with its smallerdiameter end nearer the cathode 5. The opening III in the smaller end ofthe frusto-conical or first auxiliary electrode 9 is slightly less thanthat in the control electrode 1 and the apex angle of the frusto-conicalelectrode 9 is comparatively large, of the order of for example. A shortdistance from this frusto-conical or first auxiliary electrode 9 isarranged a second frusto-conical annular electrode II also with itssmaller diameter end l2 nearer the cathode. The apex angle of thissecond auxiliary electrode is considerably less than that of the first,whilst the length of the former is preferably much greater than that ofthe latter, measured in the direction of the axis of the tube. Followingthe second auxiliary electrode is the usual anode B which isconstituted, as before, by the internal silvering on the sides of thecylindrical portion of the glass envelope.

As before, the fixed potentials applied to the electrodes increase insequence from the control electrode 1 to the anode 6, and with thisarrangement the electrostatic lines of force tend to con verge, asshown, from the large area cathode on to the axis of the tube. Sincethis concentrating eiTect extends well into the space enclosed by theanode, electrons emitted from the cathode are urged on to the axis ofthe tube and thus the apertures in the electrodes can be made ofprogressively decreasing sizes, the size of the aperture in the controlelectrode being the greatest.

Any or all of these apertures may be covered with a metallic mesh asshown in Fig. 3 in order to obtain a more perfect distribution of thelines of force.

By suitable modifications oi the sizes, dispositions and potentials ofthe control and auxiliary electrodes the grid-voltage-anode currentcharacteristic of the tube can be modified to a considerable extent andthe conditions existing between the cathode, control electrode and firstauxiliary electrode are, in fact, very similar to those existing betweenthe electrodes of an in directly heated thermionic triode valve.

If desired, more than two auxiliary frustoconical electrodes may beemployed in the tube, in which case, preferably, the apex angles ofthese electrodes decrease, and the potentials applied to them increase,with increasing distance from the cathode.

Instead of the auxiliary electrodes being of frusto-conical shape asdescribed, they may have any other suitable annular dished shape andtheir cross-sections perpendicular to the direction of the ray may beother than circular, if desired.

Having now described the invention, what I claim and desire to secure byLetters Patent is the following:

1. A cathode ray tube having an electron source, an apertured controlelectrode, at least two auxiliary electrodes each having an aperturetherein and being in the form of an annulus having a frusto-conicalshape and having its narrower end nearer the cathode of the tube and ananode electrode positioned in thetube in the order mentioned, and ascreen positioned to receive the developed electrons passed through theaforesaid electrodes to produce luminous effects.

2. A cathode ray tube having an electron source, anapertured controlelectrode, an auxil iary electrode having an aperture therein and beingin the form of an annulus having a frustoconical shape and having thenarrower end nearer the cathode of the tube and a second frustoconicalauxiliary electrode in the form of an apertured plane surface and ananode electrode positioned in the order mentioned, said electrodes beingadapted to have applied thereto potentials of values VB, Va and VD, saidpotentials thus applied to the control, frusto-conical and secondauxiliary electrodes, respectively, so as to satisfy the condition givenby the expression V V b where b represents the shortest distance betweenthe control and frusto-conical electrode and c rep-resents the shortestdistance between the frusto-conical and second auxiliary electrode.

3. A cathode ray tube having an electron source, an apertured controlelectrode, at least two apertured auxiliary electrodes and an anodeelectrode positioned within the tube in the order mentioned, each ofsaid apertured electrodes having an aperture of a size which is madeprogressively smaller in accordance with increasing distance of theelectrode from the cathode.

4. A cathode ray tube comprising an electron source, a control gridmember arranged substantially adjacent the electron source, afrustoconical auxiliary electrode member positioned with its apex towardthe electron source so that upon applying voltages between the sourceand the conical member a convergent electrostatic field serving to forcethe electrons issuing from the source into the frusto-conical member isproduced, and a frusto-conical anode electrode adapted when voltages areapplied thereto to produce jointly with the frusto-conical electrode asecond convergent electrostatic field to focus the issuing electrons toa sharply defined path, and a screen structure adapted to receive theelectron stream produced.

5. A cathode ray tube comprising an electron source, an aperturedcontrol electrode positioned adjacent the source, a wire mesh coveringsupported by the control electrode for causing a more even distributionof the lines of force between the electrodes, a first frusto-conicalelectrode member and a second frusto-conical electrode member, saidfrusto-conical electrode members having lengths. progressively varyingin accordance with the distance thereof from the electron source theelectrode most distant being of longer length, and an anode electrodearranged within the tube whereby upon the application of voltagesprogressively increasing upon all of the electrodes convergentelectrostatic fields are produced between the control grid and thefrusto-conical members to cause the electron beam to narrow and wherebybetween the anode and the frustoconical members a second convergentelectrostatic field is produced to cause the issuing electrons to assumea narrow path and to focus on a predetermined surface as a sharplydefined spot.

JAMES DWYER McGEE.

